Back in September 1981, when Roberto D'Aubuisson was still in exile in Guatemala for alleged right-wing terrorism, he made a clandestine tape recording to carry his message to the Salvadoran Army.

Like other tapes that the cashiered Army intelligence officer had sent to friends in the barracks, this one pinpointed suspected leftist sympathizers in the countryside, presumably for the benefit of the death squads with which off-duty officers kept their version of order.

But the tape had something new. In it, D'Aubuisson announced the formation of a new political party, the Nationalist Republican Alliance (known by the Spanish acronym, Arena). Despite its newness, the party could win the national elections then six months off, he said, if it took a hard line against leftist guerrillas and promised at the same time to "widen and improve" social reforms "to make the country run better."

D'Aubuisson was almost right. His simplistic message, brought to the corners of this tiny nation in a hard-driving campaign backed by wealthy Salvadoran exiles, made D'Aubuisson the president of the new legislature that came out of the March 28 election. It transformed D'Aubuisson from a charismatic young fugitive into one of the most powerful politicians in the region, a man recognized in millions of U.S. living rooms, a name to make world leaders take notice.

Of all the major actors in the Salvadoran drama, D'Aubuisson, 39, and his followers appear to stand alone in having been able to draw advantage from El Salvador's complex and dangerous conflicts:

* Leftist forces that had expected a general uprising to thwart the elections instead found 1.5 million Salvadorans swamping the polls to vote, many of them avoiding guerrilla gunfire to do so. Kept from participating by a combination of their own ideology and death threats against their leadership, leftist parties might have gained 20 to 25 percent of the popular count, judging from abstentions and nullified ballots. As it was, they appeared to suffer a major political defeat in the massive turnout.

* The Christian Democratic Party--which has helped the military run El Salvador as part of a centrist junta that originally took power in October 1979 from the forces that now back D'Aubuisson--thought its social and economic reforms had converted enough peasants to its cause to give it a victory in an honest election. The party got 40 percent of all the valid votes, but the rightists who took the remaining 60 percent were able to join forces and immediately declared victory.

* The Reagan administration also expected the elections to ratify the reforms that Washington had said were essential to a continuation of U.S. aid, but instead they legitimized D'Aubuisson's far right. Only narrowly did a combined effort by the State Department and the Salvadoran armed forces avert a total rout by the right last week, and the future division of influence here still is uncertain.

* The more independent-minded officers in the Armed Forces, who had gained a measure of dignity and pride in the past 30 months by detaching themselves and their men at least in part from the Army's old wealthy patrons and its history of bloody political intervention, have once again been forced to warn of coups if the reforms are jettisoned.

So strong is the resurgence of the right that the landed oligarchy, which bungled away its support in the church and the Army during the 1970s and lost most of its power after the 1979 coup, now is making a major bid to get that power back through a parliamentary system that it subverted for most of its history.

In short, the success of U.S. policy regarding El Salvador as the quarantine zone where communism must be stopped in this hemisphere has come to depend greatly on the behavior of a man the State Department once banned from the United States as a right-wing terrorist. This is true even though the State Department was instrumental in keeping D'Aubuisson from getting more power than he now has.

This time, in an ironic role reversal, the interventionist military men who, as a group, have tended to be painted by liberals as the villains behind death squads and torture, have almost become the progressives in this confused situation. It is upon the Army that U.S. hopes of a moderate government here now appear to depend.

"There is simply no tradition of compromise here," said a leading diplomat who was close to the postelection negotiations over the form the government should take.

"There is no center, only a right and a left," echoed D'Aubuisson's supporters as they waited outside the newly elected Constituent Assembly to cheer their hero.

"If we lose, thousands of us will flee the country. They will kill us like rats, like rabbits," said a leading Christian Democratic politician before the election.

To lose here has usually meant to flee or die. So it was that when the Christian Democrats took 40 percent of the valid votes, the four rightist parties that together won 60 percent quickly united and sought to exclude the Christian Democrats from power altogether.

The United States had not expected that when it began pushing for free elections here two years ago. Listening mostly to those in power, the State Department had come to believe the Christian Democrats' assertions that their ambitious program of land, banking and export reforms had broken the back of the old oligarchy, that they had wooed and won the 3 million Salvadoran peasants and effectively crippled the leftist guerrillas' appeal to the poor.

"People expected too much, they expected everything of the reforms," said an official at the government's Institute for Agrarian Transformation. "The politicians wanted us to finish off the guerrillas and to incorporate all the peasants into the political and economic life of the country . . . and the peasants expected that they would suddenly have money. It didn't work that way."

The election was structured to favor smaller parties, and so much attention was paid to the unprecedented effort of making the vote honest that not much attention was paid to the platforms of those groups. Few Salvadorans knew one from the other, but voters angry at the Christian Democrats for the ongoing war or the lagging reforms had somewhere to go if they could not stomach D'Aubuisson. It was not foreseen that the four opposition parties would form such a united front.

The prime victim was the man who had been most visible, President Jose Napoleon Duarte. A folk hero after the Army stole the 1972 election from him, tortured him and exiled him to Venezuela, Duarte disappointed many of his followers when he returned in 1980 as the Christian Democratic head of a military-civilian junta, praising as "noble" the Army that under previous leaders had cut off two of his fingertips.

He was a hard-line reformer, the devil incarnate to the landowners and their bankers whose privileges he tried to distribute to the poor. But he miscalculated, too, virtually ignoring the entire private sector, including small businessmen. The result was a flight of money from the country--$820 million since 1979, according to U.S. government figures--and a current investment rate of zero.

Duarte also had the bad luck to preside during a world slide of prices for the three crops that keep El Salvador alive: coffee, cotton and sugar. El Salvador has very few dollars with which to buy the fertilizer and pesticides that keep those crops coming in.

Duarte further had to prosecute a war that official figures say has blasted $100 million off the country's roads, power lines and factories. Violence has taken perhaps 30,000 lives since he took office and has displaced half a million more people.

"The right doesn't really understand all this," said a foreign economic analyst here. "They'll begin to grasp it when a businessman wants the Central Bank to spend dollars for machinery and they won't have it to give."

The economic situation is so bad now, in fact, that it could boomerang on the right. A stiff new Internatinal Monetary Fund deal is in the final stages of negotiation, but its belt-tightening measures will involve a devaluation of the colon, which is sure to upset the business community. At present the official rate of exchange is 2.5 colones to the dollar, while on the black market it takes between 3.5 and 4 colones to buy one dollar.

Taking over all this next week as provisional president of the new government is a respected banker, economist and tax expert, Alvaro Alfredo Magana, 56. A political independent, he was handpicked by the Army and all but imposed last week on the squabbling politicians who could not otherwise agree on which of them should have the job.

Magana (pronounced Ma-GAN-ya) likes to recall that he met his wife at a basketball game 30 years ago.

"I was never on the starting team, but I was the first substitute," he said in an interview before his election. Politics, he said, is like that: "There are some games when you are happier if the coach does not send you in."

Magana says he plans to be only a caretaker president, coordinating the provisional government that will write a new constitution and set up full elections within the next two years and using politicians as his chief advisers. Magana was picked, he says frankly, because he has many friends in the Army.

As the director for 17 years of the Salvadoran Mortgage Bank, the nation's largest, Magana has given out hundreds of loans to Army officers through the military credit agency. Critics call that graft, but friends say it helped free the officers from the financial clutches of the oligarchs.

Magana's friends say his pussycat style will vanish in his first face-off with D'Aubuisson, and that Magana will defend existing reforms more than adequately. But the assembly under D'Aubuisson has already voted itself veto power over most of Magana's appointments, and the balance of power between him and the assembly has yet to be determined. At the same time, Magana will be working with three vice presidents, one from each major political party, in an executive structure that appears designed for paralysis.

"Magana is transitory. What's worrisome is that the legislature goes on forever under the constitution they'll be writing," said a high official in the armed forces.

Paralysis is the one thing the Army hopes to avoid in the continuing war with guerrillas in the countryside. By the end of June, $21 million in new U.S. airplanes will be delivered here, including two large C130 transports, and 470 combat officers will be back this month from three months of training at Fort Benning, Ga. A full battalion of Green Beret-trained soldiers just returned last week from Fort Bragg, N.C., and the Army is bracing for a renewed guerrilla offensive.

The latest showing in the elections at first erased all talk of negotiating with the guerrillas, but D'Aubuisson surprised everybody two weeks ago by saying he was "willing to talk" to two underground leftist political parties, the National Revolutionary Movement and the Nationalist Democratic Union. He declined however, to call the talks negotiations and added that if a strengthened amnesty program to bring the guerrillas out of the mountains does not work, he would press forward with a military solution.

What D'Aubuisson is clearly offering is the separation of political and economic strategies against the left from the military approach.

"If the politics doesn't work, and he will make sure it won't, then he has a green light for a real war," said an academician strongly critical of D'Aubuisson. D'Aubuisson's real character remains a mystery, even as it grows more important in El Salvador's future. His inaugural speech as president of the assembly promised to guarantee human rights in El Salvador.

"I never thought I'd hear him say that," marvelled a senior diplomatic observer.